There exist many instances in the food industry where a solid food component, typically of meat, fish or vegetable, requires to be processed by heating and after cooling to be packaged. Sterilisation is an essential requirement in such procedures and is commonly effected during processing. Generally, however, the processed food component then requires both to be transferred to packaging equipment and packaged without compromising the previously achieved sterility.
It is known to at least partly overcome this problem by processing the food within its packaging. In general however, this has only been accomplished by application of heat from outside the packaging, e.g. by use of microwaves. Food processing in this way is not wholly satisfactory. AS the heat is applied from the outside inwards, the temperature rise at the outer regions of the food component is higher than that at the inner region, and unless the outer regions are substantially overprocessed, which is highly undesirable, the inner regions are not adequately processed and may not be properly sterilised, which is equally undesirable.
Especially for liquid foods, ohmic heating is a known process for effecting sterilisation. This involves passing an electric current directly through the food. With ohmic heating it is possible to carry out processing in a manner which achieves more uniform temperatures throughout the food. Prima facie, however, ohmic heating is not applicable to a food already enclosed within its packaging.
From U.S. Pat. Nos. 3,873,742 and 4,161,908, it is known to process a salt-impregnated food of kneadable nature, such as minced meat, by ohmic heating. The methods disclosed involve the filling of a tube of wrapping material which is sealed at its ends by conductive end members in contact with the food material with which the tube is packed. Current is then passed between the end members to heat the food by the Joule effect.
The methods known from these patents are not applicable to solid food components which possess a definite shape. Moreover, a disadvantage of the disclosed methods is that the end members form part of the finished food wrapping.
Having regard to all the foregoing, it is an object of the present invention to provide an improved system for processing, sterilising and packaging a solid food, especially a solid food component having a definite shape.
The invention
According to one aspect of the invention, there is provided a method of processing, sterilising and packaging foods, according to which a solid food component is located within a sealable packaging film defining a sealed enclosure, the enclosure is filled with an electrically conductive liquid, an electric current is passed through the contents of the enclosure in order to effect ohmic heating of the food component, after processing the food component the enclosure is exhausted of liquid and of air, the packaging material is allowed to collapse and is resealed to encapsulate the food component, and the encapsulated food component is separated from the remainder of the packaging film which defined the original enclosure.
The electrically conductive liquid employed is preferably one which has sterilising properties, such as a saline solution.
This method has the advantages of processing by a method which can be controlled to achieve substantially uniform temperatures throughout the solid food component, of avoiding any step of transfer to a packaging station after processing, and of sterilising the internal surface of the packaging film, which constitutes the sole sealed wrapping of the finished food, at the same time as food processing.
Our International Published Application WO/94/08475 describes a method of and apparatus for processing a solid food component by ohmic heating in a manner which achieves remarkably uniform temperatures throughout the food. In this method, the food is located in saline liquid in an enclosure in which the pressure is controlled, in relation to the saturated vapour pressure of the food, throughout ohmic heating. This method is applicable to the present invention, more particularly in that, during processing, pressure within the enclosure is controlled by restricting the expansion of the liquid.
Preferably, the packaging film is used to define an enclosure of known volume and the amount of liquid required to fill the volume is monitored to determine the volume of the food component in order to provide control data for processing by ohmic heating. Thus, the control data provides the information necessary to determine the magnitudes of the electric current and the applied pressure during ohmic heating of a particular food component.
Usually, after processing by ohmic heating and exhaustion of the liquid, the enclosure is at least partly evacuated and the food component allowed to cool prior to encapsulation. In a preferred method, on completion of processing by ohmic heating, the liquid is flushed from the enclosure by pressurised sterile air, and then evacuated to initiate the cooling process.
In a preferred method, the enclosure around the food component is defined by at least one film former and a pair of anvils to which the packaging film is sealed to define the ends of the enclosure. The anvils can carry electrodes and at least one of the anvils carries liquid and air service pipes for supply of liquid and air to and exhaustion of liquid and air from the enclosure. Conveniently, after removal of the film former or formers, the anvils may be movable towards one another to allow the packaging film to collapse when the food component is to be encapsulated.
Thus, in a preferred method, after completion of cooling in the evacuated condition, the enclosure is allowed to return to atmospheric pressure with sterile air. The film former or formers are then removed, and the air again evacuated from the enclosure at the same time as the anvils are moved towards one another. The packaging film thus collapses around the food component and can be sealed to encapsulate the food as collapse of the film takes place. As the film forming the encapsulation is a part of the film forming the original enclosure, its inner surface has been sterilised with the food during the ohmic heating process, and no additional step of sterilising the packaging is necessary.
The encapsulation can be severed from the remainder of the film forming the original enclosure as a dual action of the film sealing step which completes the food encapsulation. It then remains only to remove the encapsulated food component and prepare for processing, sterilisation and packaging of the next food component.
According to another aspect of the invention, there is provided apparatus for processing, sterilising and packaging food, comprising means defining a sealed enclosure of sealable packaging film around a solid food component, means for filling and exhausting the enclosure with an electrically conductive and sterilising liquid, means for filling and exhausting the enclosure with sterile air, electrodes for passing a current through the enclosure to effect processing of the food component by ohmic heating, means operative after processing and exhaustion of liquid and air for allowing the packaging film to collapse around the food component, and means for resealing the film as an encapsulation around the food component and for separating the encapsulated food component from the remainder of the packaging film which defined the original enclosure.
The means defining the enclosure, in addition to the packaging film, preferably comprises at least one film former and a pair of anvils which form opposite ends of the enclosure. The anvils may conveniently carry the electrodes and the means for filling and exhausting the enclosure with liquid and air.
Preferably, after exhaustion of liquid and air from the enclosure and removal of the film former or formers, the anvils are movable towards one another to enable the packaging film to collapse around the food component.
According to a preferred method, the enclosure is defined with known volume and means are provided for monitoring the amount of liquid required to fill the container in order to provide control data for processing by ohmic heating. Moreover, in accordance with the method and apparatus described in our aforementioned Europen patent application, means are provided for controlling the pressure within the enclosure during processing by ohmic heating.